Curvature of the energy per particle in neutron stars

Abstract

Neutron stars (NSs) serve as laboratories for probing strongly interacting matter at the most extreme densities. Their inner cores are expected to be dense enough to host deconfined quark matter. Utilizing state-of-the-art theoretical and multi-messenger constraints, we statistically determine the bulk properties of dense NS matter. We show that the speed of sound can be expressed in terms of the slope and curvature of the energy per particle. We demonstrate that the restoration of conformal symmetry requires changing the sign of the curvature of the bulk energy per particle as a function of energy density. Furthermore, we find that such a sign change is closely related to the peak in the speed of sound. We argue that the curvature of the energy per particle may serve as an approximate order parameter that signifies the onset of strongly coupled conformal matter in the NS core.